killing may add to immune destruction but may play a relatively minor role. Although not all scientists agree on the relative contributions of these various mechanisms to the development of AIDS, they are in agreement that there may yet be facets of HIV that remain to be uncovered. Some hope that a definitive answer might lie in these still undiscovered interactions.

Scientists are interested in the natural history of the virus in part because it is so unusual and its study promises to unlock many mysteries about the different ways that evolution has invented to do things. Take, for example, the unexpected finding of reverse transcription, which taught scientists that information could flow in more than one direction.

Beyond satisfying intellectual curiosity, the intense interest that HIV and AIDS have received is motivated by a deep desire by the scientific community to eradicate HIV and AIDS. In the past, science has eliminated plagues with vaccines and drugs, and it is looking toward these to deal with HIV and AIDS. While people can try to avoid contact with infectious agents, some infection is inevitable. Most encounters with pathogens are completely random, and while no one can control when they might run across a virus or a bacterium that will cause them to get sick, vaccines can help them prepare in advance to fight the infection before it gains access to their system.

The immune system is a defense network. Like any defense system, it needs to know what its enemy looks like, so it knows what to attack. Of course, after infection, once an invader has made its way into a person's system, the immune system will learn to recognize it and will respond by producing antibodies and deploying killer T-cells. But mobilizing such an attack takes time. The immune system needs to decipher the features of the foreigner and discern that the pathogen is in fact foreign. While the immune system is doing that, the pathogen is multiplying and causing disease.

With vaccines, physicians try to eliminate or at least reduce the lag time so that the immune system is ready to kill an invading pathogen at the moment it enters the body, before it can infect cells, multiply, or cause disease. The vaccine is a device to teach the immune system of an uninfected person the features of a pathogen before that person ever comes in contact with it. The concept is identical with hanging a "Wanted" poster in public places, so criminals will be recognized and apprehended before they commit another crime.